Off axis rotation. Not a flip, not flat. And by off-axis, it is implied that the "axis" is an orthogonal set perpendicular in one dimension and parallel in the other to the direction of gravity.

Edit, that's not really clear I guess. Basically, a spin where you rotating not around your waist or over your head. The least corked spins literally look like a corkscrew ... imagine spinning around through the air and creating a helix with your shoulders. Corks have become so "corked" that they sometimes are closer to like an underflip but are still off-axis.

Now, what do you guys think about grabs ?
I have the feeling that there is a general rule that applies for grabs to improve spin.
I ride goofy and when I spin backside I mostly grab with my right hand because it feels natural. I don't really spin frontside that good but in my head it seems easier to grab with my left hand.

So, is there a general rule to grab when spinning backside or when spinning frontside to make it easier ?

Most people prefer a grab that allows their shoulder to drop or open and see the direction of travel so they can rotate better and see the landing. So for backside usually a lead hand toeside grab like Mute or a trailing hand heelside grab like stale. For frontside typically the opposite so Indy or Melon. Thats a pretty general rule though, you will get different answers but these grabs will help you turn your head, hense they should help your rotation along.

That's a back cork 5. If you pop a little more instead of just dropping your shoulder at the lip, you'll get the full rotation. That jump is pretty lippy and should give you plenty of cork. Looks good though.

As for the general rule with grabbing, what you said is definitely true. If you grab indy or melon frontside or mute stalefish on backside rotations, it's just natural because like you said, your shoulders are already wound up in that direction. Same applies if you backside shifty grabbing melon or indy will tend to be natural, stale and mute on frontside shiftys. However, grabbing the opposite way can be challenging and look really stylish.

Here at 1:18 of this edit (Nick Poohachoff is the rider) you can see him go back 5 melon which actually harder to grab since your shoulders naturally want to wind the other way spinning backside. However, it looks sweet and when you release the grab you can see how the board snaps around because of the tension built up in the core. It's just a way to add more style and diversify your riding.